A measured difference in RNA expression level between two samples is the result of both true biological as well as experimentally induced (technical) variation. Different variables, inherent to the RT-qPCR workflow need to be controlled for in order to minimize the technical variation. Influencing parameters include the amount and quality of starting material, enzymatic efficiencies, and overall transcriptional activity.
It is highly recommended to minimize the technical variation by using standard operating procedures throughout the entire qPCR workflow. The remaining technical variation should then be further reduced or removed by using a proper normalization approach, enabling a better appreciation of the true biological variation.